In the language of graph theory, we can define a “loop" simply as a closed path on a graph with non-repeating edges or vertices (a cycle) [1]}. On a bipartite graph, the length of a loop must clearly be even, which we can denote as \(2l\)  (where \(l \ge 2\)  is a positive integer). We again resort to using the terminologies of RBM, and refer to vertices in one set as visible nodes and vertices in the other set as hidden nodes.
